The present invention relates to medical ultrasound imaging equipment and, in particular, to a biocompatible material for coupling ultrasonic energy between a patient""s skin and an ultrasonic transducer used in such equipment.
Ultrasound may be used to measure the physical characteristics of living tissue. In echo ultrasound, an ultrasonic transducer is placed against the skin to transmit sound into the patient, and then to receive echo signals caused by the reflections of the ultrasonic energy across interfaces between materials of different acoustical properties. In transmission ultrasound, an ultrasonic transmitting transducer is placed against the skin on one side of the patient to transmit sound through the patient to be received by a second ultrasonic transducer placed against the skin on the other side of the patient.
Transmission ultrasound finds considerable use in quantitative measurements of tissue characteristics and, in particular, in the field of bone densitometry where measurements of speed of sound and broadband ultrasonic attenuation (BUA) may be used to characterize bone health. Such systems are described in U.S. Pat. Nos. 4,930,511, 5,042,489, 5,054,490, 5,099,849, 5,119,820, 5,218,963, 5,343,863, 5,349,959, 5,483,965, 5,603,325, 5,840,029, 6,027,449, 6,277,076, and 6,364,837, all assigned to the assignee of the present invention and hereby incorporated by reference.
The delivery of ultrasound into the patient requires an efficient coupling path between the transducer and the patient""s skin usually facilitated by a coupling material. Such coupling materials are typically selected to be hypoallergenic, slow to dry, and of comparable acoustic qualities to water (the principal constituent of tissue). Some commonly used coupling materials are glycerol, water, and oils.
In many applications, a gel material is used because of its lessened tendency to flow from the region in which it is applied and its ability to fill gaps between the ultrasonic transducer and the skin. U.S. Pat. No. 4,002,221 describes a gel coupling agent having a viscosity similar to mayonnaise and formed from copolymers of methylvinyl-ether and maleic acid and carboxy-polymethylene polymer with alkali metal salts as thickeners. U.S. Pat. No. 4,459,854 describes a coupling material composed of a hydrogel being a copolymer of vinyl pyrrolidone and phenolethyl methacrylate. Numerous of ultrasound coupling gels are commercially available.
Precise quantitative measurements in bone densitometry studies often require a stabilization period after the coupling material has been applied to the patient and the ultrasonic transducers have been positioned. This stabilization period is about one minute with maximum stabilization occurring as much as fifteen to twenty minutes later. The mechanism underlying this need for a stabilizing period is not well understood.
The present inventor has determined that the stabilization period needed for quantitative bone density measurements is essentially eliminated (approximately fifteen seconds) when a thin coating of alcohol is used as the coupling material between the ultrasonic transducer and skin.
While the inventor does not wish to be bound by a particular theory, it is believed that the reduction of stabilization time may result from alcohol""s ability to penetrate or wet the skin very quickly, which in turn may be a function of its extremely low surface tension. In contrast, gels, for example, may tend to entrap air and impede the wetting of the skin""s surface.
Unlike gels and other coupling materials, the high volatility of the alcohol makes for easy cleanup and does not leave a residue on the patient""s skin. Alcohol has a sterilizing property, presents a low safety hazard, is easily dispensed from a spray bottle without danger of contamination, and is readily available in the hospital or clinic environment.
The invention may be particularly suitable for ultrasound systems that use an inflated bladder having a convex outer surface where little gap filling is required and where the contact area of the bladder during inflation spreads outward clearing bubbles from the region.
These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention.